Optical upconverter

ABSTRACT

An optical upconverter having a pump beam, a signal beam, and an optically nonlinear material for mixing the pump and signal beams to obtain an output beam having a frequency which is the sum of the pump and signal beams. The pump beam is generated by a laser and the nonlinear material is mounted in the laser cavity. The laser beam is a linearly polarized plane wave. The signal beam is directed into the cavity through a polarizer which linearly polarizes the signal beam in a direction which is orthogonal to the pump beam. The nonlinear material is cut and oriented in the cavity such that nonlinear mixing of the pump beam will occur only with the signal beam and not with itself. Therefore, depletion of the pump beam will occur only during mixing with the signal beam, and second harmonic generation of the pump beam will not occur.

DUI/L595 United States Patent Firester 1 Feb. 29, 1972 [54] QPTICALUPCONVERTER Primary Examiner-Roy Lake n I Assistant ExaminerDarwin R.Hostetter [72] Anhur Kendall Park Attorney-Harry M. Saragovitz, EdwardJ. Kelly, Herbert Berl [73] Assignee: The United States of America asand Jeremiah 0. Murray represented by the Secretary oi the Army 22Filed: May 15,1970 [57] ABSTRACT An optical upconverter having a pumpbeam, a signal beam, [2]] Appl' and an optically nonlinear material formixing the pump and signal beams to obtain an output beam having afrequency [52] U.S.Cl. ..307/88.3,250/83.3 HP which is the sum of thepump and signal beams. The pump ll?- Cl. r "H03! beam is gene atcd w andthe nonlinear material is [58] Field oiSearch ..307/88.3;330/4.5; moumcdin the lasacavity The beam is a linearly 250/833 HP polarized planewave. The signal beam is directed into the cavity through a polarizerwhich linearly polarizes the signal [56] References Cited beam in adirection which is orthogonal to the pump beam. The nonlinear materialis cut and oriented in the cavity such UN ST ES TEN [TED AT PA TS thatnonlinear mixing of the pump beam will occur only with 3,450,997 6/ I969Patel 307/883 the ignal beam and not with itself Therefore, depletion ofthe 3,487,230 12/1969 COStlCh ....307/88.3 ump beam will occur onlyduring mixing with the signal $300,653 1/ 1957 y at a] beam, and secondharmonic generation of the pump beam will 3,384,433 5/1968 Bloembergen......307/88.3 not occut 3,551,844 l2/l970 Smith ..307/88.3

6 Claims, 2 Drawing Figures MIXER OPTICAL UPCONVERTER The presentinvention relates to image converters and the like and more particularlyto optical upconverters using nonlinear optical materials.

In the fields of thermography, night surveillance, night communications,etc., it has been the general practice to employ devices such as imageconverters for the purpose of converting infrared images into visibleimages. The optical upconverter shifts the frequency of a relativelyweak signal beam into the visible light region, by mixing the weaksignal beam, and a relatively strong pump beam in a nonlinear opticalmaterial to obtain a visual output at the sum frequency.

Nonlinear optical upconverters have not proved entirely satisfactory forthe reason that depletion of the pump beam by nonlinear interactions ofthe pump beam with itself have attributed to severe inefficiencies.

The general purpose of this invention is to provide a nonlinear opticalupconverter wherein depletion of the pump beam occurs only by the mutualinteraction of the pump and signal beams. To attain this, the presentinvention contemplates a unique optical system having a nonlinearmaterial oriented such that the nonlinear interaction of the two beamsoccurs only when each of them has a well defined and mutually orthogonalpolarization.

It is, therefore, the primary object of the present invention to providea highly efficient optical upconverter.

Another object is to provide an optical upconverter having an output andonly if both the pump and signal beams are present.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of a preferred embodiment of theinvention; and

FIG. 2 is a vector diagram helpful in describing the invention.

Referring now to the drawing there is shown in FIG. 1 an opticalupconverter having a laser 11 for providing a pump beam (0,, a signalbeam (0,, and a nonlinear optical material 13. laser 11 and nonlinearmaterial 13 are mounted in a resonant cavity formed by minors l4 and 15,polarizer 16, and dichroic mirror 17. Laser 11 is represented simply bya box and the specific elements such as the laser pump, the coolants,etc., are not shown for purposes of simplicity.

The nonlinear material 13 must be transparent at the laser frequency 0,.The laser beam at, is a linearly polarized plane wave. The polarizer 16therefore reflects all of the beam to, toward the nonlinear material 13.The signal beam to, enters the resonant cavity through the polarizer 16which linearly polarizes the signal beam at, in a direction which isorthogonal to the laser beam (0,, Le, one beam being the ordinary rayand the other being the extraordinary ray.

The nonlinear material 13 is chosen such that an upconverted signalwl-m, will be produced in the crystal due to nonlinear interactionsbetween the signal arrd pump beams in, and 0,. Therefore, as a result ofthe upconversion an infrared image having a center frequency to, may beconverted into a visible image having a center frequency w,+u,.

Generally the principles upon which the nonlinear material 13 mixes twobeams is well known and is described in the literature. One basicrequirement is that the nonlinear material 13 be index matched for thefrequencies to, and 01,. Index matching is the process by which theelectric fields of all three waves (0,, (0,, and w,+m, are made totravel in material 13 at the proper velocities so that the mixing canoccur in phase over the thickness of the material 13. The velocities ofthe waves (0,, 1a,, and m,+m, must sa tisfy the wavevector equation I+II Ir or for three colinear waves n ,(ar,-l1u,)=n,w,+n,m,where n, 11,,and n, are the indices of refraction, taken into account thepolarization directions, of the sum, pump, and signal wavesrespec,ively.

Under some circumstances index matching cannot be achieved, therebyrequiring that the material 13 be longitudinally very thin with respectto the wavelength of the radiation being mixed.

Besides generating the sum frequency u,+u,, the output of a nonlinearmaterial can also contain frequencies (0, and 0,, the difierencefrequency w,w,, and higher harmonics of u, and in, due to self-mixing aprocess which can cause unwanted outputs and depictions of the pump beamor,

However, in accordance with the principles of the present invention,depletion of the laser beam or, occurs only during interactions with thesignal beam or, and not with interactions with itself, i.e., higherhannonics of the pump will not be generated as a result of self-mixing.Therefore, depletion of the laser beam or, will be a minimum and willtake place only when the signal beam w, is present.

This feature is provided in the present invention by choosing for mixer13 a nonlinear optical crystal having a symmetry such that thecomponents of the nonlinear polarizability tensor which couple parallelcomponents along one of the crystal axes all vanish. The crystal is thenout such that this axis lies in the plane of the major flat surface ofthe crystal and is oriented parallel to the electric field of the pumpbeam (0,, which is linearly polarized by polarizer 16.

The electric polarization vector produced by the nonlinear mixing ofinput electric fields is given by the tensor equation:

where the d are the nonlinear susceptibilities which forms the nonlinearpolarizability tensor. Theoretically, a material is nonlinear, if anyone or more of the d do not vanish. The symmetry of the crystal latticedetermines which ones of the d do vanish. All noncentrosymmetriccrystals are capable of nonlinear mixing and all crystals in the sameclass have the same d s equal to zero. However, the absolute magnitudeof the d which do not vanish are generally different and must bemeasured for each crystal.

For example, the following expressions are true for all crystals in theclass 12m:

P: nl y z r znr z w In the above expressions the subscripts x, y and 2represent the natural crystallographic axes. The KDP crystal is anexample of a crystal in this class. Therefore, the polarization vector Pfor all crystals in the class 32m is defined by the same generalequations as that for KDP. It can, therefore, be seen that in order forthe crystal to radiate along a particular axis, one must first have apair of mutually orthogonal electric field components, and the crystalmust be cut such that one of the major flat surfaces thereof isperpendicular to that axis. For example, if there are two input electricfields having components E, and 5,, then there will be a polarization P,and the crystal will radiate along an axis which is perpendicular to amajor flat surface of the crystal (it is assumed that the flat surfaceis not perpendicular to the x-axis).

If the nonlinear material 13 is cut and oriented as shown in FIG. 2 andbelongs to the class of crystals 42m, then depletion of the pump beamto, will take place only when the signal beam or, is present. In FIG. 2,the orthogonal set of axis .r, y and 2 represent the normalcrystallographic axes. An xz-cut 12m crystal 13 is oriented with respectto the pump and signal beams w, and m, such that the major faces ofcrystal 13 is perpendicular to the Z axis, which is the direction ofpropagation of the pump and signal beams in, and w,. Polarizer 16 hasinsured that the pump and signal beams in, m, are linearly polarizedsuch that the corresponding electric fields E(w,) and E00,) lie alongaxes which are mutually orthogonal. Also, the crystal 13 is orientedsuch that the y and y axis are colinear and the electric field E(w,) ispolarized along the y' or y axis. As a result, the electric vector5(0),), which is polarized along the x axis and therefore has acomponent along the x-axis, produces a polarization component P,(w,+r inaccordance with the last of the above expressions, i.e., P,

= d,,,,,. Further, the polarization vector P,(w,+w,) will have acomponent P,1(w,+ui,) along the x-axis which is parallel to the majorface of the crystal 13. Finally, there will then be radiation of afrequency w, (0,, the electric field of which will be polarized alongthe direction of the the polarization vector P,-

The nonlinear susceptibilities which give rise to self-mixing of a fieldoriented along one of the axis x, y or z are all zero in this case.Therefore, if there is no signal beam (0,, then there will be nononlinear mixing and the pump beam in, will simply pass through thecrystal 13 without any depletion.

Another class of suitable crystals, clas 33m, has the followingrelationship between the polarization and the electric fields:

An example of a crystal in this class is GaP. It is to be noted, thatupconversion can take place here without self-depletion of the pump beam(0,, if the crystal is given a yz-cut, the pump beam to, is linearlypolarized with the electric vector (0),) polarized parallel to thex-axis, and the signal beam (a, polarized parallel to the yz-plane.Other cuts and orientations are also possible.

An example of a crystal class not suitable for practicing this inventionis the class 3m. Lithium niobate is a typical crystal in this classand-is characterized by the following expressions.

P: zn r z r wz v z P: uetx am av zu z Although the pump and signal beamson, and m, can mix to produce a sum beam at, a), as a result of thefirst two equations, there will also be a contribution to the Pcomponent from the E, or 5,, components as well as the E, componentswhich give rise to self mixing This will produce a radiation field atthe second harmonic of the pump beam 0),, thereby depleting the pumpbeam 1, even when the signal beam in, is not present.

It should be understood, of course, that the foregoing dis closurerelates to a preferred embodiment of the invention and that numerousmodifications and alterations may be made therein without departing fromthe spirit and the scope of the invention as set forth in the appendedclaims.

What is claimed is:

1. An optical frequency converter comprising pump means for generating apump signal; polarizing means for linearly polarizing said pump signalparallel to a first axis; input means for linearly polarizing an inputsignal along a second axis perpendicular to said first axis; a nonlinearmixing means mounted in the path of said pump signal and said inputsignal and including a nonlinear optical crystal having a polarizability tensor of nonlinear susceptibility components with those nonlinearcomponents associated with the self mixing of a field polarized alongsaid first axis substantially equal to zero and having a substantialnonlinear component for mixing a signal polarized along said first axiswith a signal polarized along said second axis.

2. The device according to claim 1 and wherein said pump means includesa laser.

3. The device according to claim 2 and wherein said nonlinear opticalcrystal is located in the resonant cavity of said laser.

4. An optical frequency converter comprising a laser means having aresonant cavity means for generating a linearly polarized pump s gnal;said cavity means including a signal input means for linearly polarizingan input signal in a direction perpendicular to said pump signal; anonlinear mixing means mounted in said cavity for mixing said pumpsignal only with said input signal to produce a mixed signal; and acavity output means for passing said mixed signal out of said cavity.

5. The device according to claim 4 and wherein said mixed signal has afrequency which is the sum of said pump signal frequency and said inputsignal frequency.

6. The device according to claim 5 and wherein said nonlinear mixingmeans includes a nonlinear optical crystal having a polarizabilitytensor of nonlinear susceptibility components; and said tensor havingthose components associated with the self-mixing of field componentspolarized in the direction of said pump signal substantially equal tozero.

1. An optical frequency converter comprising pump means for generating apump signal; polarizing means for linearly polarizing said pump signalparallel to a first axis; input means for linearly polarizing an inputsignal along a second axis perpendicular to said first axis; a nonlinearmixing means mounted in the path of said pump signal and said inputsignal and including a nonlinear optical crystal having a polarizabilitytensor of nonlinear susceptibility components with those nonlinearcomponents associated with the self mixing of a field polarized alongsaid first axis substantially equal to zero and having a substantialnonlinear component for mixing a signal polarized along said first axiswith a signal polarized along said second axis.
 2. The device accordingto claim 1 and wherein said pump means includes a laser.
 3. The deviceaccording to claim 2 and wherein said nonlinear optical crystal islocated in the resonant cavity of said laser.
 4. An optical frequencyconverter comprising a laser means having a resonant cavity means forgenerating a linearly polarized pump signal; said cavity means includinga signal input means for linearly polarizing an input signal in adirection perpendicular to said pump signal; a nonlinear mixing meansmounted in said cavity for mixing said pump signal only with said inputsignal to produce a mixed signal; and a cavity output means for passingsaid mixed signal out of said cavity.
 5. The device according to claim 4and wherein said mixed signal has a frequency which is the sum of saidpump signal frequency and said input signal frequency.
 6. The deviceaccording to claim 5 and wherein said nonlinear mixing means includes anonlinear optical crystal having a polarizability tensor of nonlinearsusceptibility components; and said tensor having those componentsassociated with the self-mixing of field components polarized in thedirection of said pump signal substantially equal to zero.